1. Field of the Invention
The present invention relates to a dipole coil and, in particular, a dipole coil including saddle-shaped coils forming the constituent element of a superconducting electromagnet.
2. Description of the Related Art
Superconducting electromagnets formed of a dipole coil structure including saddle-shaped coils are known in the prior art. One such saddle-shaped coil and the elements associated therewith in the dipole coil structure are illustrated in FIG. 1.
In this figure, reference numeral 11 designates a saddle-shaped coil, reference numeral 3 designates a straight portion spacer, reference numerals 5 designate end plates, respectively, and reference numeral 6 designates inner spacer structure not shown in any particular detail.
The saddle-shaped configuration of coil 11 is constituted by both linear central portions of the coil, one of which is designated by reference 11a, and curved saddle portions 11b of the coil located at opposite ends of the coil, respectively. These curved saddle portions 11b extend contiguously with the central portions 11a from locations at which the linearly extending central portions of the coil begin to assume a curved configuration.
In the fabrication of the dipole coil structure of the prior art, wire of a superconducting alloy is wound around spacer structure 6 over an inner lining of a cylindrical form (not shown) to thereby form the saddle-shaped coil 11. Subsequently, the straight portion spacers 3 are butted against the sides of the linear central portions 11a of the coil with such a force as to cause a compression of the linear central portions of the coil in the direction indicated by arrow A. This direction corresponds to the circumferential direction of the inner lining on which the coil 11 is wound. End plates 5 are butted against the curved saddle portions 11b and therefore, generally compress the saddle portions 11b in the direction indicated by arrows B (axial direction of the inner lining). The purpose of precompressing the coil 11 in the manner described above will now be explained.
When the superconducting electromagnet employing the structure described above is operated, an electromagnetic force is generated which acts on the coil itself. This electromagnetic force tends to displace various windings of the coil relative to one another. Such relative displacements of the coil in turn create friction giving rise to increases in temperature at various local portions of the coil. These increases in temperature can take the alloy of the coil outside its critical temperature range whereupon the coil loses its superconducting capability at the above-mentioned local portions. Such a condition represents the germination of so-called quenching.
That is, when the local portions of the coil no longer exhibit superconductivity due to the temperature increases thereof, the local portions conduct in an ordinary manner and thus necessarily generate some quantity of ohmic heat (Joule's heat). This ohmic heat has an additive effect on the above-described temperature increases resulting in further quenching.
Accordingly, it was desired to prevent the relative displacement of the coil windings in the prior art by the use of the above-described spacers 3 and end plates 5.
However, it is difficult to confine (precompress) each local portion of the coil, in a manner which will prevent relative displacement of the windings thereof, due to the saddle-shaped configuration of the coil. In this respect, it should be noted that the maximum electromagnetic force acts at that location on the saddle-shaped coil where the linearly extending central portions 11a of the coil begin to assume the curved configuration of the saddle portions 11b.
As discussed above, in the prior art superconducting electromagnet, the end plates 5 are effective to precompress the saddle portions 11b of the coil in the axial direction of arrows B.
However, as should now readily be appreciated, these end plates 5 exert substantially no compressive force on the coil at those locations at which the linearly extending central portions 11a of the coil begin to assume the curved configuration of the saddle portions 11b. In other words, the axial force in the direction of arrows B have substantially no component which will act to compress the coil 11 in a direction perpendicular to the windings thereof at those portions of the end plates 5 butting against the ends of the spacers 3.
And, since it is at these locations where the maximum electromagnetic force acts on the coil itself, each curved portion of the coil where a central portion 11a and a saddle portion 11b merge is a place where quenching frequently occurs.
In addition, it may be considered to use further straight spacers at the ends of the central spacer 3 so as to exert compressive forces in the direction of arrow A on those portions of coil where the linear central portions 11a begin to assume the curved configuration of the saddle portions 11b. However, such a solution would produce a sharp step between the central spacer 3 and the further straight spacers. Not only would such a step define a small space where absolutely no compressive force would be exerted on the coil, but such a sharp step would provide poor insulation and could also damage the wire or cable of the coil.